Gas chromatography (GC) is a central analytical technique that serves a broad range of applications. In GC the sample is typically injected in a mixture with a few or many other matrix compounds, and separated in time by the GC column before its detection and quantitative determination by the GC detector. As a result, the GC separation capability is defined as one of its most important (prime) capabilities. Currently, most GC analyses are performed with a 30 m long non-polar capillary column with 0.25 mm ID, operated with 1 ml/min carrier gas flow rate, and the separation is predominantly based on the sample volatility, so that the elution times follow the boiling temperatures (order of volatility) of the eluting sample compounds. While the most widely used GC detector is the flame ionization detector (FID), the most important GC detector is clearly the mass spectrometer. The combination of gas chromatography and mass spectrometry (GC-MS) is a known powerful technology since GC-MS, unlike GC-FID, can also be used for sample identification, and it excels in mass selective detection of low level compounds in complex matrices. For a variety of petrochemical, food and other complex samples the GC separation is insufficient, due to lack of peak capacity and limited separation capability, and mass spectral identification is often hampered by extensive co-elution of several compounds that prohibit appropriate identification and quantitative determination. In order to improve the GC separation, comprehensive two-dimensional gas chromatography (GCxGC) was pioneered by Phillips and Liu [U.S. Pat. Nos. 5,135,549 and 5,196,039] and later on was further developed by Phillips and co-workers and many other investigators. GCxGC is based on comprehensive two dimensional GC separation, so that the full sample is first separated according to its volatility (boiling point) on typically a non-polar standard column and then in repeated cycles, the eluting compounds are focused (usually cryo focused) in space and pulsed injected into a second short semi polar or polar column for their second dimension separation according to the sample compounds polarity. GCxGC sample compounds that co-elute from the first analytical column typically have similar boiling points. Upon their pulsed injection into a second short column with polar separation film, those compounds that are chemically polar have stronger interactions with the second column adsorption film, are adsorbed for a longer period of time and thus the second column order of elution follows the sample order of polarity. GCxGC recently became a popular research area and it is well described in a few review articles such as J. Dalluge, J. Beens and U. A. T. Brinkman, J. Chromatogr. A. 1000, 69-108 (2003). GCxGC provides the following main advantages: a) Improved GC separation by a factor of about 5 to 20; b) Improved sensitivity by a factor of about 2-7 in view of 4 to 50 times narrower GC peaks; c) Reduced matrix interference in view of the improved GC separation, and d) Provision of additional sample polarity and group type information according to its second column order of elution.
The main added method and hardware element in GCxGC versus GC is the sample modulator. In GCxGC, the sample compounds which elute from the first column, are focused in space and pulse injected into a second short column for their second dimension separation. The sample focusing and pulsed injection into the second column is performed with a device called a sample modulator or modulator in short. The modulator determines the efficiency, performance, reliability and added cost of GCxGC and is hence considered the most important element in GCxGC technology. Several distinctive designs of GCxGC modulators were developed and some are commercially available. Currently, thermal modulators, which are based on sample cryogenic focusing (cryo-focusing), are most abundant and widely used as they provide the best GCxGC resolution through narrowest second GCxGC column injection time width. All the thermal modulators, however, are complex and costly devices, further consuming large amounts of CO2 gas or liquid nitrogen, in the order of three large CO2 cylinders per day or 80-100 liters of liquid nitrogen per day. As a result, despite the superior GCxGC performance over standard GC, its use and applicability are limited, in view of the cost and complexity of GCxGC thermal modulators and their high gas consumption.
While GCxGC was proven to be a powerful technique for improved separation of complex mixtures, most of the research was performed with GCxGC-FID and not with GCxGC-MS. Whenever sample identification and/or confirmation are required, however, the use of mass spectrometry for detection is mandatory. Since GCxGC with the common thermal modulators generates narrow chromatographic peaks with peak width that could be below 0.1 s, GCxGC requires the fast scanning speed of time of flight (TOF) mass spectrometers. GC-TOF-MS is, however, expensive and not as robust as standard quadruple GC-MS, and thus, GCxGC-TOF-MS is not frequently used. In view of the high cost and complexity of GCxGC-TOF-MS, GCxGC with quadruple MS was successfully explored and proved to be a valuable approach, although faster scan speed is still a highly desirable feature.
As a result of the above, GCxGC-MS suffers from several problems that need to be solved in order to make it a generally accepted useful analytical tool:    1. There is a need to solve the problem of adequate scan speed for quadruple MS, in order to enable the proper combination of GCxGC with the robust and low cost range of quadruple MS (or ion trap). Modern quadruple GC-MS are characterized by improved scan speed of >8,000 amu/s that can be >20 Hz in the 50-450 amu mass range.    2. Currently samples are identified mostly via the use of 70 eV EI libraries. However, sample identification with libraries is limited since most of the organic compounds are not included in any library. The identification of unknown compounds can be further confronted by frequent absence (or weakness) of the molecular ion.    3. The problem of frequently absent or weak molecular ions is further exacerbated with TOF-MS for two main reasons: a) TOF-MS uses a multi channel ion detector and do not employ a conversion dynode for reasons of time resolution, so that in GC-TOF-MS, the molecular ion and other high mass ions are relatively suppressed in comparison with quadruple GC-MS that uses conversion dynode based ion detectors, and b) while it is recognized that narrow GCxGC peaks require fast scan speed, it is similarly true although generally neglected that narrow GCxGC peaks similarly require fast ion source response time. Semi-volatile and low volatility samples, however, tend to have lengthy intra ion source adsorption-thermal-desorption cycles, which induce peak broadening and tailing. Thus, the need for 10 times faster ion source response time, also requires ˜70° C. hotter ion source temperature. This increased ion source temperature significantly (exponentially) reduces the relative abundance of the molecular ion and impede sample identification both with and without the library.    4. The known Achilles Hill of GC is its limited range of thermally stable and volatile compounds amenable for analysis. This problem of limited range of compounds amenable for analysis is further exacerbated in GCxGC-MS due to the need to use significantly increased ion source temperature, which promote excessive intra ion source decomposition of thermally labile compounds.    5. Thermal modulators are complex, expensive and require large amount of cryogenic gases.
Thus, there is a long felt need to provide a quadrupole (or ion trap) based GCxGC-MS that will effectively operate without requiring costly cryogenic gases and that will provide trustworthy molecular ion combined with library searchable mass spectra for an extended range of thermally labile and low volatility samples.
Around the year 2000, Seeley and co-workers, (J. V. Seeley, F. Kramp and C. J. Hicks, Anal. Chem. 72, 4346-4352 (2000)), developed and presented a new approach of flow modulation GCxGC as an alternative to thermal modulation GCxGC and later on they further characterized and explored this flow modulation method. According to Seeley's flow modulation method, the output of the first GCxGC analytical column is directed to a storage transfer line column and then injected with typically 20 times higher flow rate (20 ml/min) into the second GCxGC column, while the first analytical column output flow is directed to a second parallel storage transfer line column. After a few seconds the 20 times higher flow rate is modulated and directed to the second storage transfer line column for the injection of its content into the second GCxGC column and such a cycle is repeated for affecting comprehensive GCxGC analysis. Flow modulation is a relatively simple and low cost method for GCxGC modulation, but it suffers from two new problems:    A) Flow modulation GCxGC is characterized by significantly reduced GCxGC separation due to the use of high column flow rate of 20 ml/min in combination of large bore capillary column (such as with 0.32 mm I.D.). The combination of 3 meter 0.32 mm I.D. column operated with 20 ml/min, provides about 20 times less number of theoretical separation plates in comparison with the typically used 1 meter long 0.1 mm I.D. micro-bore capillary columns. This reduced number of separation plates can be translated into 4.5 times poorer GCxGC separation, which serves as a major deterrent from flow modulation GCxGC.    B) Flow modulation requires compatibility with a high second column flow rate of about 20 ml/min, which makes it incompatible with standard GC-MS instrumentation that are designed to work with 1 ml/min column flow rate. Although some GC-MS systems can accept a few ml/min column flow rates, their ion source response is significantly lower with higher column flow rates due to increased intra ion source space charge.
The combination of A and B above and the need to address the problem of limited scan speed of standard mass analyzers (quadrupole and ion trap) explains why despite its cost and maintenance advantages, flow modulation GCxGC was not combined with standard GC-MS.
In the last decade there has been developed and explored the performance capabilities of a new type of GC-MS, based on the use of a supersonic molecular beam (SMB). SMB was used for interfacing the GC to the MS and as a medium for ionization of sample compounds while vibrationally cold in the SMB, either by electron ionization (EI) or by hyperthermal surface ionization (HSI). SMB (with helium as carrier gas) is characterized by intra-molecular vibrational super-cooling of its seeded sample molecules due to relatively low collision energies of sample compounds and carrier gas species during the supersonic expansion. Consequently, the molecular ion (M+) intensity is enhanced in EI with SMB (also named “cold EI”) and it is practically always exhibited, yet the library searchable fragment ions are retained. In addition, isomer mass spectral information is significantly amplified and accurate isotope abundance information is revealed (which also provide unique elemental formula information) without any self-chemical ionization interferences. The SMB interface is compatible with high carrier gas flow rate of about 90 ml/min through the use of differential pumping in an added vacuum chamber. Furthermore, SMB is characterized by the fly-through motion of the sample compounds in the ion source and by vacuum background filtration, and as a result, the SMB fly-through ion source is characterized by tailing-free ultra-fast ion source response time regardless the sample volatility, which is an important feature for GCxGC-MS in view of its need to comply with the tailing free analysis demands of sub one second GC peaks. Thus, the use of GC-MS with SMB provides a range of advantages which offset its higher price and added complexity in view of the need for added vacuum chamber and pump.
While evaluating the flow modulation method, it is realized that an important yet not fully appreciated feature of flow modulation is that the minimal GCxGC peak width (injection time) is the collection time (typically 4 sec.) divided by the second column to first column flow rate ratio (typically 20). Thus, flow modulation provides GCxGC peak width that can be controlled, including in the convenient range of 0.2-0.3 s in order to make it amenable for the scan speed of quadruple or ion trap GC-MS. As a result, it was surprisingly found that the previously considered adverse flow modulation feature of increased peak width, hence lost GCxGC separation resolution has an important merit in enabling the use of quadruple mass spectrometry (and other low scan speed MS such as ion trap and magnetic sector) despite its limited scan speed. On the other hand, in order to benefit from this feature, it is required to address the standard GC-MS flow rate limitation of 1 ml/min and enable the use of 20 ml/min carrier gas flow rate without any sacrifice in sensitivity. The solution for this demanding feature is the use of GC-MS with supersonic molecular beam according to the present application that can accept any column flow rate up to 100 ml/min without any adverse effect on its sensitivity.
Consequently, in flow modulation GCxGC-MS with SMB, the problem of limited quadruple mass analyzer scan speed is solved via flow modulation injection broadening of the eluting peaks, which as a result of their increased peak width require less mass spectral scans per unit time. In addition, the molecular ion abundance is significantly enhanced through the use of SMB, ultra-fast ion source response time is provided with SMB regardless of the sample volatility, and the flow modulation requirement of compatibility with 20 ml/min column flow rate is easily addressed by the SMB differential pumping.
Flow modulation seems deceivingly simple but it was found that it possesses several limitations and disadvantages which hampers its effective use including:    1. The Seeley flow modulation “arrangement” is delicate. It is composed of four T union connectors plus six “small” transfer line columns in addition to the GCxGC two main analytical columns, and this complete structure has to be mounted on a piece of mesh introduced into the limited space of the GC oven;    2. The flow modulation structure contains four delicate low thermal mass T union connectors that are the subject of frequent leaks;    3. The Seeley flow modulation GCxGC method development is relatively complicated, as it is based on narrow range of flow rates and flow impedance transfer line columns. As a result, once the flow modulation is based around 1 ml/min first column flow rate, it is inconvenient and not practical to change this flow rate;    4. Flow modulation GCxGC must use constant pressure and is incompatible with the GC industry standard mode of constant flow operation unless an additional electronic flow control is used;    5. Flow modulation GCxGC is characterized by reduced second dimension GC separation. The same desirable feature of relatively broad GCxGC peaks implies lost GCxGC resolution. In addition, the high second column flow rate also implies reduced number of second column separation plates and peak capacity, and    6. The Seeley flow modulation method shares with thermal modulation GCxGC the problem of ghost peaks (also named turn around peaks) in the second analytical column.